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# BMAT section 2 q help watch

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1. Can someone please explain to me why the answer to this is C and not D??
2. I think you have to recognise that there is background radiation (or something of the sort that isn't coming from the source) which has a count rate of 20 (which is why you have a straight line at 20 counts/min for detector 2). This means you wouldn't look at half of 220, instead, you'd look at half of 200, as that is the initial count rate of the radiation from the source, and then you'd add on 20 counts/min from the external radiation, to find the point where the count rate of the source has halved.
So, half of 200 is 100, plus 20, gives you 120 - this comes at 2.4 hours
3. (Original post by iras)
I think you have to recognise that there is background radiation (or something of the sort that isn't coming from the source) which has a count rate of 20 (which is why you have a straight line at 20 counts/min for detector 2). This means you wouldn't look at half of 220, instead, you'd look at half of 200, as that is the initial count rate of the radiation from the source, and then you'd add on 20 counts/min from the external radiation, to find the point where the count rate of the source has halved.
So, half of 200 is 100, plus 20, gives you 120 - this comes at 2.4 hours
ohh okay thank you! I was halving 220 and then taking into account the 20 counts..

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